1. Technical Field
The present disclosure relates to a damping force control device for a vehicle such as an automobile.
2. Description of the Related Art
In a vehicle such as an automobile, a shock absorber is disposed between a sprung mass and an unsprung mass corresponding to each wheel. Each shock absorber damps vertical vibration of a sprung mass by generating a damping force corresponding to a product of a relative speed of the sprung and unsprung masses and a damping coefficient. Since a required damping force varies depending on a running condition of a vehicle, damping force variable type shock absorbers capable of changing damping coefficients are mounted depending on the vehicle, and the damping coefficients are controlled according to the running condition of the vehicle.
Vertical vibrations of the sprung mass generated when a vehicle is traveling include multiple vibrations of various frequencies. Particularly, when a vehicle travels on a running road having a waviness, vertical vibrations of the sprung mass are vibrations having relatively low frequencies and large amplitudes, that is, vibrations with many so-called floating components. On the other hand, when a vehicle travels on a running road with an irregular road surface, vertical vibrations of the sprung mass are vibrations having relatively high frequencies and small amplitudes, that is, vibrations with many so-called rugged components.
When the vertical vibrations of the sprung mass are vibrations with a relatively larger amount of floating components, it is preferable that a damping force be increased in order to effectively attenuate the vertical vibrations of the sprung mass to improve a ride comfort of the vehicle. On the other hand, when the vertical vibrations of the sprung mass are vibrations with a relatively larger amount of rugged components, if the damping forces are increased, the vibrations of unsprung masses received from a road surface are easily transmitted to the sprung mass so that the ride comfort of the vehicle is deteriorated. Consequently, the damping forces are preferably lowered. Therefore, it is preferable that the damping forces are controlled in accordance with a ratio of floating components and rugged components contained in the vertical vibrations of the sprung mass.
For example, Japanese Patent Application Laid-open No. H08-216646 discloses a method of extracting floating components and rugged components from a vertical acceleration of a sprung member, calculating an increase/decrease amount of a damping force based on a maximum value of absolute values of each component extracted during a predetermined period of time, and increasing and decreasing the damping force of each shock absorber based on the calculated increase/decrease amount. In calculating the increase/decrease amount of the damping force, a map showing the relationship between the maximum value of the absolute values of the floating component and the rugged component and the increase/decrease amount of the damping force is referred to.
In the damping force control device described in the aforementioned Japanese Patent Application Laid-open No. H08-216646, a map must be produced for each type of vehicle having different specifications, by extracting floating components and rugged components from the vertical acceleration of the sprung mass and calculating a necessary increase/decrease amount of the damping force based on maximum values of the absolute values of the components that are extracted during a predetermined period of time. A map is a two-dimensional map showing the relationship between the maximum values of the absolute values of floating components and rugged components and the necessary increase/decrease amount of the damping force and it is necessary to determine the necessary values experimentally, for example. Consequently, it takes a lot of time and labor to produce a map.